1. Field of the Invention
The present invention relates to communication devices and high-frequency couplers for large-capacity data transmission in close proximity through a weak UWB communication technique using a high-frequency wide band and, in particular, to a communication device and high-frequency coupler with a communicable area being sufficiently extended in a lateral direction in weak UWB communications using electric-field coupling.
2. Description of the Related Art
Non-contact communication has been widely available as a medium for authentication information, electronic money, and other value information. Examples of an IC card standard complying with ISO/IEC 14443 include Type A, Type B, and FeliCa®. Furthermore, near field communication (NFC) developed by Sony Corporation and Koninklijke Philips Electronics N.V. is an RFID standard mainly defining specifications of an NFC communication device (reader/writer) communicable with an IC card of each of Type A, Type B, and FeliCa. In NFC, with the use of a band of 13.56 MHz, non-contact bidirectional communication of a close-proximity type (0 or more but 10 or less cm) can be performed through electromagnetic induction.
An example of a further application of a non-contact communication system is large-capacity data transmission, such as downloading or streaming of moving pictures and music. For example, it can be assumed that non-contact communication is used when a fee-based content is downloaded from a vending machine to a portable terminal or when a content is downloaded from a fee-based site to a portable terminal via a personal computer connected to the Internet. In this case, it is preferable that a single user operation of holding the portable terminal over a read surface suffices and the operation completes with the sense of the same access time as that for authentication and billing in the past. Therefore, a high communication rate is desired.
However, the communication rate in NFC communication, which is a typical example of non-contact communication, is approximately 106 kbps to 424 kbps, which is sufficient for personal authentication or billing process but is extremely slow compared with other general-purpose wireless communication (such as WiFi and Bluetooth). Also, in the technique in the past, such as NFC communication, an achievable maximum communication rate is up to 848 kbps at best, due to physical constraints, such as carrier frequency. Thus, it is difficult to expect a dramatic increase in rate in the future.
By contrast, an example of a close-proximity wireless transfer technology applicable to high-speed communication is TransferJet using a weak ultra wide band (UWB) signal (for example, refer to Japanese Unexamined Patent Application Publication No. 2008-99236 and www.transferjet.org/en/index.html (as of Mar. 23, 2009)). This close-proximity wireless transfer technology (TransferJet) is of a technique of basically transmitting a signal by using an electric-field coupling action, and a communication device for the technique includes a communication circuit unit processing a high-frequency signal, a coupling electrode disposed so as to be separated from a ground at a predetermined height, and an resonating unit efficiently supplying the high-frequency signal to the coupling electrode.
In weak UWB communications using an electric-field coupling action, its communication distance is approximately 2 cm to 3 cm, and the high-frequency coupler does not have a polarized wave but has a directivity of approximately the same extent both in a height direction and in a lateral direction. Therefore, the communication device using the high-frequency coupler has a substantially hemispherically domed communicable area.
On the other hand, in an NFC communication technique, a small-sized and low-height reader/writer module of a compact size suitable for incorporation has been developed and manufactured, and can be used as being implemented for use in various devices, such as a point of sales (POS) terminal, vending machine, and personal computer. For example, an information processing device of a notebook type is suggested in which a reader/writer module is incorporated in a palm rest portion of a keyboard of a main body and information is read from a non-contact IC tag nearby (for example, refer to Japanese Unexamined Patent Application Publication No. 2003-87263).
In the above-described usage pattern in which a reader/writer module is implemented in a device, a unit serving as a communication target, such as a portable terminal, is generally placed on a read surface of the device so as to advance into a communicable area. Now consider the case where this general practice is applied to the above-described weak UWB communication technique. With a portable terminal having a high-frequency coupler incorporated therein being placed on a read surface of the device, the distance between the high-frequency couplers is short, and the high-frequency coupler is in an approximately contact state. In this manner, the high-frequency coupler can be sufficiently close to the device to the extent of generating an electric-field coupling action occurs. Originally, the high-frequency coupler has a directivity of approximately the same extent both in a height direction and in a lateral direction (as described above). Even when the communication distance in the height direction (that is, a signal propagating direction) is short, if the communicable area is extended in the lateral direction (in other words, a direction orthogonal to the propagating direction), an area where the read surface, that is, the communication device, can be placed is extended, thereby making it easier for users to use. Since the high-frequency coupler is incorporated in a different place for each terminal, the distance between the high-frequency couplers varies even if each terminal is placed on the read surface of the device in a similar manner.
For example, a communication system of a weak UWB communication technique is suggested in which a surface-wave transmission line made of a linear member using a copper line or other conductors is disposed near a coupling electrode and an electric-field signal emitted from a coupling electrode of a transmitter propagates through the inside and surface of this transmission line to extend the distance between the electrodes (for example, refer to Japanese Unexamined Patent Application Publication No. 2008-99234). This system uses a property that the electric field proceeds along the surface of a conductor. That is, an electric field perpendicular to the surface-wave transmission line and a magnetic field winding around the outside of the surface-wave transmission line occur and, with the form of energy alternately changing between the electric field and the magnetic field, a surface-wave signal propagates far away.
However, the above-described communication system using a surface-wave transmission line extends the communication distance in the height direction (that is, the signal propagating direction), and does not extend the communicable area in the lateral direction (in other words, the direction orthogonal to the propagating direction). Also, along a longitudinal direction of the surface-wave transmission line, a strong portion and a weak portion periodically occur in the electric field so as to correspond to a maximum position and a node of a standing wave. Although users can intuitively know that communication quality is higher at places closer to the center of the read surface, the center of the surface-wave transmission line in a longitudinal direction does not necessarily have a high electric field strength (refer to Japanese Unexamined Patent Application Publication No. 2008-99234, FIGS. 31A to 31F). Therefore, it is difficult for the user to perform an intuitive operation.
As another example, a surface-wave transmission line is suggested having one end formed of a bundle of line materials with their end face attached to a coupling electrode at an appropriate position and angle for an excellent communication state and the other end unbundled with an end face of each of the line materials distributed over a flat coupling surface (for example, refer to Japanese Unexamined Patent Application Publication No. 2008-103993). With this, a coupling surface having a communicable area extended in the lateral direction can be provided. However, it is difficult to make a small-sized high-frequency coupler of low height via a surface-wave transmission line made of a bundle of line materials.